diff --git a/oidc_provider/middleware.py b/oidc_provider/middleware.py
index 511851e..a359a95 100644
--- a/oidc_provider/middleware.py
+++ b/oidc_provider/middleware.py
@@ -12,6 +12,6 @@ class SessionManagementMiddleware(MiddlewareMixin):
     """
 
     def process_response(self, request, response):
-        session_state = sha224(request.session.session_key or django_settings.SECRET_KEY).hexdigest()
+        session_state = sha224((request.session.session_key or django_settings.SECRET_KEY).encode('utf-8')).hexdigest()
         response.set_cookie('op_browser_state', session_state)
         return response